Many modern speech coders belong to a large class of speech coders known as LPC (Linear Predictive Coders). Examples of coders belonging to this class are: the 4,8 Kbit/s CELP from the U.S. Department of Defense, the RPE-LTP coder of the European digital cellular mobile telephone system GSM, the VSELP coder of the corresponding American system ADC, as well as the VSELP coder of the Pacific Digital Cellular system PDC.
These coders all utilize a source-filter concept in the signal generation process. The filter is used to model the short-time spectrum of the signal that is to be reproduced, whereas the source is assumed to handle all other signal variations.
A common feature of these source-filter models is that the signal to be reproduced is represented by parameters defining the output signal of the source and filter parameters defining the filter. The term "linear predictive" refers to a class of methods often used for estimating the filter parameters. Thus, the signal to be reproduced is partially represented by a set of filter parameters.
The method of utilizing a source-filter combination as a signal model has proven to work relatively well for speech signals. However, when the user of a mobile telephone is silent and the input signal comprises the surrounding sounds, the presently known coders have difficulties coping with this situation, since they are optimized for speech signals. A listener on the other side may easily get annoyed when familiar background sounds cannot be recognized since they have been "mistreated" by the coder.